In the area of liquid level sensing devices, mechanical or float systems are the most widely used. These devices when coupled with levers, relays, and switches can be used to provide a shutoff signal when the liquid falls below a minimum level. The reliability of these systems is limited and, therefore, have been replaced in some installations by electrical level sensing systems. These systems generally rely upon the attenuation of an electrical signal by the conductive liquid to activate a controlled load such as a heater. Failure of this type control system will result in the controlled load being continuously activated when the liquid falls below the minimum level.
Other systems which rely upon the conduction of 60 cycle leakage currents through the conductive fluid to activate a controlled load suffer from erratic control indications due to 60 cycle hum pickup in the probe wiring. Other available conducting systems of the D.C. type cause electrolysis of the conducting liquid due to the passage of a D.C. current through the liquid.